1. Field of the Invention
This invention relates to circuit protective devices and, more particularly, to fuse-type resistive devices applicable for direct mounting on printed circuit boards.
2. Description of the Prior Art
One common fuse-type protective device frequently employed heretofore incorporates a simple fusible wire element that is typically maintained under spring tension within an insulative housing, and which element melts or softens sufficiently when subjected to a predetermined current-induced overload condition so as to break and establish an open-circuit condition. One of the earliest and simplest of this type of fuse is the so-called fust plug, one of which is disclosed in R. S. Porter U.S. Pat. No. 1,340,080.j Such plugs typically have a tapered threaded end that is simply screwed into a socket-receiving electrical receptacle. A variation of that type of fuse, having fixedly secured metal end caps, and adapted for spring clip mounting, is disclosed in C. L. McAlister U.S. Pat. No. 2,913,555. Neither of the above-described prior art devices, of course, is adapted to employ leads not only to provide the means for interconnecting the device with associated circuitry, but to normally facilitate holding the assembled device together, and to provide the means for directly mounting the device on, and optionally (with pre-formed stops) in spaced relationship with, a circuit board, for example.
Neither of the above-mentioned types of circuit protective devices likewise incorporates a resistive element as part of the internal circuit path thereof. One prior protective device that does incorporate a resistor is disclosed in A. Haynman U.S. Pat. NO. 2,966,649. The resistor in that device is not enclosed within an insulative housing, nor does it function as a fusible element. Rather, an open-circuit condition is dependent on the melting of an external solder joint that connects a folded-back end portion of one of the resistor leads with a terminal that forms part of a bracket coaxially mounted on the resistor. The leads also have no preformed bends that would or could facilitate the direct mounting of the device on a circuit board, or that could function as inherent stops so as to position the resistor itself above the circuit board with a predetermined space therebetween. Such spacing is often desired in order to protect associated circuitry from any detrimental heat that could possibly develop should the protective device malfunction for any reason.
In still another prior resistive protection device particularly employed in the telephone industry, the resistor is mounted within an insulative housing, with one internal lead thereof being permanently secured to one conductive housing end cap, with the other lead being solder-connected to an internal elongated member that is normally maintained under spring-biased tension while secured to an opposite end stationary, conductive, housing end cap. An open-circuit condition in this latter device is also dependent on the melting of the internal soldered connection of predetermined dimensions. As this latter device has fixedly secured conductive end caps, it is not adapted for direct mounting, such as by the resistor leads, within suitably aligned thru-holes of a circuit board. There likewise is no means to space the housing of the device a predetermined distance above a circuit board as a safety factor.
From the foregoing, it is seen that neither of the described resistive type fuse-protective devices effect an open-circuit condition, in response to a given current-induced overload condition, without the need of a soldered connection as the fusible element. Moreover, the leads thereof are configured and secured in such a manner that they are neither readily adapted for direct circuit board securement, nor are they adapted for used as mounting elemnts with stops for spacing the body of the device above a circuit board.
With respect to resistive type protection devices, in particular, it would also be desirous to enclose the latter within an insulative housing in a manner that not only protects associated circuitry from possible heat damage, but provides a means for readily assembling and disassembling the device without the need of any soldered connections, fastening screws or threaded members. There is also a definite need for a resistive element that not only functions as the fusible element per se, but that is capable of withstanding momentary d-c and a-c voltage surges of relatively high magnitude, such as of the order of hundreds of volts, for time durations ranging from a fraction of a second to one or several seconds, without impairment, while still being very sensitive to very low steady state currents, such as on the order of milliamperes in magnitude.